Recently, rare earth element-containing compounds have been investigated for permanent magnets. Such magnets are currently being produced for various applications. For example, a high energy product, high coercivity permanent magnet composition comprising iron, neodymium and/or praseodymium and boron and the methods of making the same are disclosed in U.S. Pat. No. 4,496,395 issued Jan. 29, 1985 and U.S. Pat. No. 4,802,931 issued Feb. 7, 1989, both to John J. Croat and assigned to the assignee of this application. In those patents, magnetically hard compositions having high values of coercivity, remanence and energy product contain rare earth elements, transition metal elements and boron in suitable proportions. Melt-spun ribbons of the permanent magnetic material are disclosed as well as methods of making the same.
Furthermore, U.S. Pat. No. 4,792,367 issued Dec. 20, 1988 to Robert W. Lee, again assigned to the assignee of this application, discloses a high energy product, magnetically anisotropic permanent magnet which is produced by hot working quenched or fine grained, melt-spun materials comprising iron, neodymium and/or praseodymium and boron to produce a fully densified, fine grained body which has undergone plastic flow.
In the past, samarium cobalt magnets were the preferred permanent magnets. However, low availability of the raw materials has prompted research into other arenas of material science. Consequently, magnets having grains of a predominant atomic formula of Nd.sub.2 Fe.sub.14 B.sub.1 are being investigated. These magnets may be made from more readily available materials, without substantially sacrificing the magnetic performance. Thus, great initiative has been placed into researching ways and methods of optimizing the performance of the Nd.sub.2 Fe.sub.14 B.sub.1 magnetic materials.
In order to make neodymium-iron-boron magnetic materials commercially feasible, there are certain problems which require attention. These include (1) increasing the useful service life of the material and (2) rejuvenating materials which have degraded over time after exposure to atmospheric conditions. In addition, neodymium-iron-boron magnets have traditionally shown a somewhat low crush strength and a relatively intolerant resistance to corrosion. Consequently, these problems have been investigated and addressed by the present inventors during the course of their research.
The conventional neodymium-iron-boron magnets are limited in that they exhibit decreased magnetic properties over time due to the degradation of the materials. They have exhibited a relatively low crush strength and a commensurate low resistance to corrosion. The methods which have been used to produce such magnets had been acceptable, although there was room for improvement. The current research has been directed towards increasing the remanent moment, Br, and the intrinsic coercivity, Hci, of the magnetic materials.
It has been recognized that neodymium-iron-boron magnetic materials may be produced by melt spinning, such as the materials which are disclosed in the above-mentioned U.S. Pat. No. 4,496,395 issued Jan. 25, 1985 to John J. Croat. The materials referenced above in U.S. Pat. No. 4,802,931 entitled "High Energy Product Rare Earth-Magnet Alloys" relates to alloying mixtures of one or more transition metals and one or more rare earth elements. The alloys are quenched from a molten state at a controlled rate and thereafter solidified to produce finely grained crystalline microstructures.
As discussed above, it would be advantageous to restore the magnetic properties of these melt-spun materials lost due to degradation over time. It would also be advantageous to increase the crush strength of the resulting magnets as well as to increase the resistance to corrosion. Therefore, it is an object of our invention to provide a method of treatment for the magnetic compositions to effect a partial revival of the original magnetic properties of the material, to increase the crush strength and to increase the corrosion resistance of the material.
It is another object of our invention to provide methods for enhancing the properties of the magnetic materials utilizing techniques which are simple, relatively quick, non-toxic, and particularly effective. It is also to be recognized that the cost of such a method must be sufficiently low such that it is economically feasible to utilize such a method without adding substantially to the overall lifetime cost of the permanent magnet.